3D-CFD modelling of combustion and knock risk assessment in a high-performance gasoline direct injection engine

Compression ratio and boost pressure increment represent an effective solution to enhance performances and thermal efficiency of turbocharged spark-ignition internal combustion engines (ICEs). Nonetheless, the higher in-cylinder pressure and temperature may favor the onset of knocking phenomena, thus limiting the achievable potential improvements. The high number of engine operating parameters on which it is possible to act make experimental campaigns extremely time and capital intensive. Therefore, numerical simulations could represent a valuable solution for analyzing the combustion process, thus predicting knock occurrence by reducing the number of experimental tests and lead time. In this context, the aim of this activity is to develop a 3D-CFD numerical model able to predict the occurrence, location and onset of knock in a high-performance turbocharged gasoline direct injection (GDI) spark-ignition engine, with a limited computational time required differently from other approaches available in literature. With this aim, a statistical knock analysis on the experimental data was carried out to identify the knock-limited spark advance condition under two high-speed and high-load working points. Then, a Reynolds-Averaged Navier-Stokes (RANS) 3D-CFD model was developed on CONVERGE CFD and validated against experimental data. Combustion was modelled through the detailed chemistry solver (SAGE). To this purpose, the CONVERGE Zero-Dimensional Chemistry Tool was exploited for defining the composition of a ternary surrogate with chemical and physical properties in agreement with the gasoline used for the experimental tests. Simulation results show good agreements with experimental data. Finally, a methodology for the estimation of a knock-risk index, based on the concentration of a chemical species (CH2O) inside the end-gases, was applied to this case study, to identify the regions inside the combustion chamber that are more prone to knock.